Wide field of view (WFOV) optical systems, such as reflective afocal telescopes are used in a variety of applications, such as multispectral applications and certain space sensor systems, for example. All-reflective optical systems are desirable for many wideband optical applications because they reflect all wavelengths of incident light equally, unlike refractive systems wherein the refraction is wavelength dependent. The object space field of view may be viewed at the exit pupil by various types of sensors which include an imaging (focal) optical system and a detector, such as a focal plane array.
Conventional afocal WFOV reflective optical systems include the three mirror anastigmat (TMA) telescope and the four mirror anastigmat (4MA) telescope. The three mirror afocal design provides usable object space field of view (FOV) coverage in two distinct regimes: 1) at high magnifications (>10×) and utilizing an on-axis primary mirror, the useable FOV is limited to about 1.0-1.5 degrees, and 2) at moderate magnifications (4-5×) and utilizing an off-axis primary mirror, the useable FOV is limited to about 2.0-2.5 degrees. U.S. Pat. No. 3,674,334 provides examples of all-reflective afocal three mirror anastigmats. The four mirror afocal design provides improvements over the TMA, but performance metrics, such as image quality, distortion, and pupil aberrations, limit its usefulness to about 5-6 degrees circular FOV. Additionally, most four mirror afocal anastigmat optical designs can be excessively large in size. U.S. Pat. No. 4,804,258 provides an example of an all-reflective afocal four mirror anastigmat.